Customers may notice that when an interface is down, the " show ethernet-switching interface " output always displays "blocked by STP" for the interface, which could be misinterpreted as indicating a spanning tree issue.

This article clarifies that such an output detail is seen because legacy EX Series switches are designed so. It also clarifies that "blocked by STP" does not indicate an issue with the STP.

In Juniper legacy EX Series switches, if an interface has been configured with the spanning tree protocol but remains in down status, the " show ethernet-switching interface " output will display " blocked by STP " as shown below:

root@s02-39> show ethernet-switching interfaces ge-0/0/1 
Interface    State  VLAN members        Tag   Tagging  Blocking 
ge-0/0/1.0   down   v100                100   untagged blocked by STP

root@s02-39> show interfaces terse ge-0/0/1  
Interface               Admin Link Proto    Local                 Remote
ge-0/0/1                 up    down
ge-0/0/1.0               up    down eth-switch

This kind of output might mislead customers into assuming that the interface is down due to a problem with the spanning tree.

In non-legacy switches, the output does not show "blocked by STP" as shown below:

Output from QFX5100

{master:0}
root@qfx5100> show ethernet-switching interface xe-0/0/1    
Routing Instance Name : default-switch
Logical Interface flags (DL - disable learning, AD - packet action drop,
                         LH - MAC limit hit, DN - interface down,
                         MMAS - Mac-move action shutdown,  AS - Autostate-exclude enabled,
                         SCTL - shutdown by Storm-control, MI - MAC+IP limit hit)

Logical         Vlan                   TAG   MAC    MAC+IP STP         Logical          Tagging
interface       members                      limit  limit  state       interface flags
xe-0/0/1.0                                   294912 8192               DN               untagged 
                default                1     294912 1024   Discarding                   untagged 

{master:0}
root@qfx5100> show interfaces terse xe-0/0/1 
Interface               Admin Link Proto    Local                 Remote
xe-0/0/1                up    down
xe-0/0/1.0              up    down eth-switch

Output from EX4300

{master:0}
root@EX4300> show ethernet-switching interface ge-0/0/5    
Routing Instance Name : default-switch
Logical Interface flags (DL - disable learning, AD - packet action drop,
                         LH - MAC limit hit, DN - interface down,
                         MMAS - Mac-move action shutdown,  AS - Autostate-exclude enabled,
                         SCTL - shutdown by Storm-control, MI - MAC+IP limit hit)

Logical         Vlan                   TAG   MAC    MAC+IP STP         Logical          Tagging
interface       members                      limit  limit  state       interface flags
ge-0/0/5.0                                   65535  8192               DN               untagged 
                default                1     65535  1024   Discarding                   untagged 

{master:0}
root@EX4300> show interfaces terse ge-0/0/5   
Interface               Admin Link Proto    Local                 Remote
ge-0/0/5                up    down
ge-0/0/5.0              up    down eth-switch

This kind of output is the default from the " show ethernet-switching interface " command while an interface is in Down state on legacy EX switches. It is recommended that customers check the physical link and work with Support to determine and troubleshoot the actual cause for the interface to be down.